Abstract. For the first time, the direct radiative effect (DRE) of aerosols on solar
radiation is computed over the entire Mediterranean basin, one of the most
climatically sensitive world regions, using a deterministic spectral
radiation transfer model (RTM). The DRE effects on the outgoing shortwave
radiation at the top of atmosphere (TOA), DRETOA, on the
absorption of solar radiation in the atmospheric column, DREatm,
and on the downward and absorbed surface solar radiation (SSR),
DREsurf and DREnetsurf, respectively, are computed
separately. The model uses input data for the period 2000–2007 for various
surface and atmospheric parameters, taken from satellite (International
Satellite Cloud Climatology Project, ISCCP-D2), Global Reanalysis projects
(National Centers for Environmental Prediction – National Center for
Atmospheric Research, NCEP/NCAR), and other global databases. The spectral
aerosol optical properties (aerosol optical depth, AOD, asymmetry parameter,
gaer and single scattering albedo, ωaer), are
taken from the MODerate resolution Imaging Spectroradiometer (MODIS) of NASA
(National Aeronautics and Space Administration) and they are supplemented by
the Global Aerosol Data Set (GADS). The model SSR fluxes have been
successfully validated against measurements from 80 surface stations of the
Global Energy Balance Archive (GEBA) covering the period 2000–2007.

A planetary cooling is found above the Mediterranean on an annual basis
(regional mean DRETOA = −2.4 W m−2). Although a planetary
cooling is found over most of the region, of up to −7 W m−2, large
positive DRETOA values (up to +25 W m−2) are found over North
Africa, indicating a strong planetary warming, and a weaker warming over the
Alps (+0.5 W m−2). Aerosols are found to increase the absorption of
solar radiation in the atmospheric column over the region
(DREatm = +11.1 W m−2) and to decrease SSR (DREsurf = −16.5 W m−2 and
DREnetsurf−13.5 W m−2) inducing thus significant atmospheric
warming and surface radiative cooling. The calculated seasonal and monthly
DREs are even larger, reaching −25.4 W m−2 (for DREsurf). Within
the range of observed natural or anthropogenic variability of aerosol
optical properties, AOD seems to be the main responsible parameter for
modifications of regional aerosol radiative effects, which are found to be
quasi-linearly dependent on AOD, ωaer and gaer.